SQL injection is a code injection technique, used to attack data-driven applications, in which nefarious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker).[1] SQL injection must exploit a security vulnerability in an application's software, for example, when user input is either incorrectly filtered for string literalescape characters embedded in SQL statements or user input is not strongly typed and unexpectedly executed. SQL injection is mostly known as an attack vector for websites but can be used to attack any type of SQL database.

SQL injection attacks allow attackers to spoof identity, tamper with existing data, cause repudiation issues such as voiding transactions or changing balances, allow the complete disclosure of all data on the system, destroy the data or make it otherwise unavailable, and become administrators of the database server.

In a 2012 study, it was observed that the average web application received 4 attack campaigns per month, and retailers received twice as many attacks as other industries.[2]

SQL injection (SQLI) was considered one of the top 10 web application vulnerabilities of 2007 and 2010 by the Open Web Application Security Project.[5] In 2013, SQLI was rated the number one attack on the OWASP top ten.[6] There are four main sub-classes of SQL injection:

This form of SQL injection occurs when user input is not filtered for escape characters and is then passed into an SQL statement. This results in the potential manipulation of the statements performed on the database by the end-user of the application.

The following line of code illustrates this vulnerability:

statement = "SELECT*FROMusersWHEREname='" + userName + "';"

This SQL code is designed to pull up the records of the specified username from its table of users. However, if the "userName" variable is crafted in a specific way by a malicious user, the SQL statement may do more than the code author intended. For example, setting the "userName" variable as:

' OR '1'='1

or using comments to even block the rest of the query (there are three types of SQL comments[13]). All three lines have a space at the end:

' OR '1'='1' --
' OR '1'='1' ({
' OR '1'='1' /*

renders one of the following SQL statements by the parent language:

SELECT*FROMusersWHEREname=''OR'1'='1';

SELECT*FROMusersWHEREname=''OR'1'='1'-- ';

If this code were to be used in an authentication procedure then this example could be used to force the selection of every data field (*) from all users rather than from one specific user name as the coder intended, because the evaluation of '1'='1' is always true.

The following value of "userName" in the statement below would cause the deletion of the "users" table as well as the selection of all data from the "userinfo" table (in essence revealing the information of every user), using an API that allows multiple statements:

While most SQL server implementations allow multiple statements to be executed with one call in this way, some SQL APIs such as PHP's mysql_query() function do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn't stop them from modifying queries.

This form of SQL injection occurs when a user-supplied field is not strongly typed or is not checked for type constraints. This could take place when a numeric field is to be used in a SQL statement, but the programmer makes no checks to validate that the user supplied input is numeric. For example:

statement := "SELECT*FROMuserinfoWHEREid=" + a_variable + ";"

It is clear from this statement that the author intended a_variable to be a number correlating to the "id" field. However, if it is in fact a string then the end-user may manipulate the statement as they choose, thereby bypassing the need for escape characters. For example, setting a_variable to

1;DROP TABLE users

will drop (delete) the "users" table from the database, since the SQL becomes:

Blind SQL Injection is used when a web application is vulnerable to an SQL injection but the results of the injection are not visible to the attacker. The page with the vulnerability may not be one that displays data but will display differently depending on the results of a logical statement injected into the legitimate SQL statement called for that page. This type of attack has traditionally been considered time-intensive because a new statement needed to be crafted for each bit recovered, and depending on its structure, the attack may consist of many unsuccessful requests. Recent advancements have allowed each request to recover multiple bits, with no unsuccessful requests, allowing for more consistent and efficient extraction. [14] There are several tools that can automate these attacks once the location of the vulnerability and the target information has been established.[15]

One type of blind SQL injection forces the database to evaluate a logical statement on an ordinary application screen. As an example, a book review website uses a query string to determine which book review to display. So the URLhttp://books.example.com/showReview.php?ID=5 would cause the server to run the query

SELECT*FROMbookreviewsWHEREID='Value(ID)';

from which it would populate the review page with data from the review with ID 5, stored in the table bookreviews. The query happens completely on the server; the user does not know the names of the database, table, or fields, nor does the user know the query string. The user only sees that the above URL returns a book review. A hacker can load the URLs http://books.example.com/showReview.php?ID=5OR1=1 and http://books.example.com/showReview.php?ID=5AND1=2, which may result in queries

respectively. If the original review loads with the "1=1" URL and a blank or error page is returned from the "1=2" URL, and the returned page has not been created to alert the user the input is invalid, or in other words, has been caught by an input test script, the site is likely vulnerable to a SQL injection attack as the query will likely have passed through successfully in both cases. The hacker may proceed with this query string designed to reveal the version number of MySQL running on the server: http://books.example.com/showReview.php?ID=5ANDsubstring(@@version,1,INSTR(@@version,'.')-1)=4, which would show the book review on a server running MySQL 4 and a blank or error page otherwise. The hacker can continue to use code within query strings to glean more information from the server until another avenue of attack is discovered or his or her goals are achieved.[16][17]

Second order SQL injection occurs when submitted values contain malicious commands that are stored rather than executed immediately. In some cases, the application may correctly encode an SQL statement and store it as valid SQL. Then, another part of that application without controls to protect against SQL injection might execute that stored SQL statement. This attack requires more knowledge of how submitted values are later used. Automated web application security scanners would not easily detect this type of SQL injection and may need to be manually instructed where to check for evidence that it is being attempted.

An SQL injection is a well known attack and easily prevented by simple measures. After an apparent SQL injection attack on Talktalk in 2015, the BBC reported that security experts were stunned that such a large company would be vulnerable to it.[18]

With most development platforms, parameterized statements that work with parameters can be used (sometimes called placeholders or bind variables) instead of embedding user input in the statement. A placeholder can only store a value of the given type and not an arbitrary SQL fragment. Hence the SQL injection would simply be treated as a strange (and probably invalid) parameter value.

In many cases, the SQL statement is fixed, and each parameter is a scalar, not a table. The user input is then assigned (bound) to a parameter.[19]

A straightforward, though error-prone way to prevent injections is to escape characters that have a special meaning in SQL. The manual for an SQL DBMS explains which characters have a special meaning, which allows creating a comprehensive blacklist of characters that need translation. For instance, every occurrence of a single quote (') in a parameter must be replaced by two single quotes ('') to form a valid SQL string literal. For example, in PHP it is usual to escape parameters using the function mysqli_real_escape_string(); before sending the SQL query:

$mysqli=newmysqli('hostname','db_username','db_password','db_name');$query=sprintf("SELECT * FROM `Users` WHERE UserName='%s' AND Password='%s'",$mysqli->real_escape_string($username),$mysqli->real_escape_string($password));$mysqli->query($query);

This function prepends backslashes to the following characters: \x00, \n, \r, \, ', " and \x1a. This function is normally used to make data safe before sending a query to MySQL.[20]
There are other functions for many database types in PHP such as pg_escape_string() for PostgreSQL. The function addslashes(string $str) works for escaping characters, and is used especially for querying on databases that do not have escaping functions in PHP. It returns a string with backslashes before characters that need to be quoted in database queries, etc. These characters are single quote ('), double quote ("), backslash (\) and NUL (the NULL byte).[21]
Routinely passing escaped strings to SQL is error prone because it is easy to forget to escape a given string. Creating a transparent layer to secure the input can reduce this error-proneness, if not entirely eliminate it.[22]

Integer, float or boolean, string parameters can be checked if their value is valid representation for the given type. Strings that must follow some strict pattern (date, UUID, alphanumeric only, etc.) can be checked if they match this pattern.

Limiting the permissions on the database login used by the web application to only what is needed may help reduce the effectiveness of any SQL injection attacks that exploit any bugs in the web application.

For example, on Microsoft SQL Server, a database logon could be restricted from selecting on some of the system tables which would limit exploits that try to insert JavaScript into all the text columns in the database.

In February 2002, Jeremiah Jacks discovered that Guess.com was vulnerable to an SQL injection attack, permitting anyone able to construct a properly-crafted URL to pull down 200,000+ names, credit card numbers and expiration dates in the site's customer database.[23]

On November 1, 2005, a teenaged hacker used SQL injection to break into the site of a Taiwanese information security magazine from the Tech Target group and steal customers' information.[24]

On January 13, 2006, Russian computer criminals broke into a Rhode Island government website and allegedly stole credit card data from individuals who have done business online with state agencies.[25]

In January 2008, tens of thousands of PCs were infected by an automated SQL injection attack that exploited a vulnerability in application code that uses Microsoft SQL Server as the database store.[30]

In July 2008, Kaspersky's Malaysian site was hacked by the "m0sted" hacker group using SQL injection.

In 2008, at least April through August, a sweep of attacks began exploiting the SQL injection vulnerabilities of Microsoft's IIS web server and SQL Server database server. The attack does not require guessing the name of a table or column, and corrupts all text columns in all tables in a single request.[33] A HTML string that references a malwareJavaScript file is appended to each value. When that database value is later displayed to a website visitor, the script attempts several approaches at gaining control over a visitor's system. The number of exploited web pages is estimated at 500,000.[34]

In December 2009, an attacker breached a RockYou plaintext database containing the unencrypted usernames and passwords of about 32 million users using an SQL injection attack.[36]

On July 2010, a South American security researcher who goes by the handle "Ch Russo" obtained sensitive user information from popular BitTorrent site The Pirate Bay. He gained access to the site's administrative control panel and exploited a SQL injection vulnerability that enabled him to collect user account information, including IP addresses, MD5password hashes and records of which torrents individual users have uploaded.[37]

From July 24 to 26, 2010, attackers from Japan and China used an SQL injection to gain access to customers' credit card data from Neo Beat, an Osaka-based company that runs a large online supermarket site. The attack also affected seven business partners including supermarket chains Izumiya Co, Maruetsu Inc, and Ryukyu Jusco Co. The theft of data affected a reported 12,191 customers. As of August 14, 2010 it was reported that there have been more than 300 cases of credit card information being used by third parties to purchase goods and services in China.

Over a period of 4 hours on April 27, 2011, an automated SQL injection attack occurred on Broadband Reports website that was able to extract 8% of the username/password pairs: 8,000 random accounts of the 9,000 active and 90,000 old or inactive accounts.[44][45][46]

On June 1, 2011, "hacktivists" of the group LulzSec were accused of using SQLI to steal coupons, download keys, and passwords that were stored in plaintext on Sony's website, accessing the personal information of a million users.[47][48]

In June 2011, PBS was hacked, most likely through use of SQL injection; the full process used by hackers to execute SQL injections was described in this Imperva blog.[49]

In February 2013, a group of Maldivian hackers, hacked the website "UN-Maldives" using SQL Injection.

On June 27, 2013, hacker group "RedHack" breached Istanbul Administration Site.[54] They claimed that, they’ve been able to erase people's debts to water, gas, Internet, electricity, and telephone companies. Additionally, they published admin user name and password for other citizens to log in and clear their debts early morning. They announced the news from Twitter.[55]

On November 4, 2013, hacktivist group "RaptorSwag" allegedly compromised 71 Chinese government databases using an SQL injection attack on the Chinese Chamber of International Commerce. The leaked data was posted publicly in cooperation with Anonymous.[56]

On February 2, 2014, AVS TV had 40,000 accounts leaked by a hacking group called @deletesec [57]

On February 21, 2014, Hackers of a group called @deletesec hacked Spirol International after allegedly threatening to have the hackers arrested for reporting the security vulnerability. 70,000 user details were exposed over this conflict.[59]

On March 7, 2014, officials at Johns Hopkins University publicly announced that their Biomedical Engineering Servers had become victim to an SQL injection attack carried out by an Anonymous hacker named "Hooky" and aligned with hacktivist group "RaptorSwag". The hackers compromised personal details of 878 students and staff, posting a press release and the leaked data on the internet.[60]

In October 2015, an SQL injection attack was used to steal the personal details of 156,959 customers from British telecommunications company TalkTalk's servers, exploiting a vulnerability in a legacy web portal[63]

Unauthorized login to web sites by means of SQL injection forms the basis of one of the subplots in J.K. Rowling's novel The Casual Vacancy, published in 2012.

An xkcd cartoon involved a character Robert'); DROP TABLE students;-- named to carry out a SQL injection. As a result of this cartoon, SQL injection is sometimes informally referred to as 'Bobby Tables'.[64][65]

In 2014, an individual in Poland legally renamed his business to Dariusz Jakubowski x'; DROP TABLE users; SELECT '1 in an attempt to disrupt operation of spammers’ harvesting bots.[66]

Companies House, the UK's official register of companies, has a company named ; DROP TABLE "COMPANIES";-- LTD[67]

The 2015 game Hacknet has a hacking program called SQL_MemCorrupt. It is described as injecting a table entry that causes a corruption error in a SQL database, then queries said table, causing a SQL database crash and core dump.

^Microsoft. "SQL Injection". Archived from the original on August 2, 2013. Retrieved August 4, 2013. SQL injection is an attack in which malicious code is inserted into strings that are later passed to an instance of SQL Server for parsing and execution. Any procedure that constructs SQL statements should be reviewed for injection vulnerabilities because SQLi Server will execute all syntactically valid queries that it receives. Even parameterized data can be manipulated by a skilled and determined attacker.

^Imperva (July 2012). "Imperva Web Application Attack Report"(PDF). Archived(PDF) from the original on September 7, 2013. Retrieved August 4, 2013. Retailers suffer 2x as many SQL injection attacks as other industries. / While most web applications receive 4 or more web attack campaigns per month, some websites are constantly under attack. / One observed website was under attack 176 out of 180 days, or 98% of the time.